Details:

This has been fixed by the Xorg developers in commit
61502107a30d64f991784648c3228ebc6694a032
(https://cgit.freedesktop.org/xorg/xserver/commit/?h=server-1.19-branch&id=61502107a30d64f991784648c3228ebc6694a032).
This has been fixed in Cucumber Linux by the patch
http://mirror.cucumberlinux.com/cucumber/cucumber-1.0/source/x-base/xorg-server/patches/CVE-2017-12183_61502107a30d64f991784648c3228ebc6694a032.patch